Local area networks use a network cable or other media to link stations on the network. Each local area network architecture uses media access control (MAC) enabling network interface cards at each station to share access to the media.
A full duplex environment has been proposed for Ethernet networks, referred to as IEEE 802.3x, Full Duplex with Flow Control-Working Draft (0.3). The full duplex environment provides a two-way, point-to-point communication link between two network elements, for example a network station and a switched hub. Hence, two or more stations can simultaneously transmit and receive Ethernet data packets between each other via a switched hub without collisions.
Full-duplex operation does not require that transmitters defer, nor that they monitor or react to receive activity, as there is no contention for a shared medium in this mode. Full-duplex operation can be used when the physical medium is capable of supporting simultaneous reception and transmission (fibre or copper), there are exactly two stations on the link and both stations have been configured to use full duplex links. The most common configuration envisioned for full-duplex operation consists of a multiport bridge (a switch) with dedicated point-to-point connections to several end-stations.
Network congestion occurs if a receiving network element is unable to receive data at a rate greater than or equal to the transmission rate of the transmitting element. For example, traffic in a client-server environment is dominated by client requests followed by a burst of frames from the server to the requesting client. Although the full duplex environment enables the server to transmit packets while receiving requests from other clients, only a limited number of client requests can be output to the server from the switched hub at the assigned switching port. If the number of client requests exceeds the capacity of the server's port, some of the data packets will be lost. Alternatively, a client having limited buffer space may be unable to keep up with the transmission rate of the server, resulting in lost packets.
Flow control has been proposed to reduce network congestion, where a sending station temporarily suspends transmission of data packets. A proposed flow control arrangement for a full duplex environment, referred to as IEEE 802.3x[2], specifies generation of a flow control message, for example a PAUSE frame. A transmitting station that receives the PAUSE frame enters a pause state in which no frames are sent on the network for a time interval specified in the PAUSE frame. The PAUSE frame relieves congestion at the receiver. For example, in a switch with several 10 Mbps or 100 Mbps full-duplex ports, it is possible for the traffic from all the ports to overload the switch. In these periods, the switch will transmit PAUSE frames to those 10 Mbps or 100 Mbps ports that the switch believes are the source of the congestion. These stations will stop transmitting frames for the period specified by the PAUSE frame, thus relieving congestion at the switch.
The round-trip link delay between the switch and the end station has importance in times of congestion. If the link delay between the switch and the end station is long, and the bandwidth of the link is high, the transmission of a PAUSE frame after congestion is detected will not have effect until at least one round-trip link delay's worth of data has entered the switch. Similarly, when congestion is relieved and the switch transmits a PAUSE frame with value 0 (allowing station transmission), it will be at least one round-trip delay before data flows into the switch again.